Dienophiles unactivated alkynes

For a few combinations of less reactive dienes and dienophiles, transition metal catalyzed variants of the Diels Alder reaction have been developed. An example is the cycloaddition of an unpolar diene and an unactivated alkyne however, except when the reaction is catalyzed with iron, nickel, cobalt, or rho-dium(I) complexes, the temperature required often causes competing decomposition, even for the intramolecular version. [2] Wilkinson s catalyst [3] - tris(triphenylphosphane)rho-dium(I) chloride - frequently used for hydrogenations and for decarbonylations, permits the cyclization of 4 to the annelated cyclo-hexadiene 5 in excellent yield in only 15 minutes at 55 °C in trifluoroethanol as solvent (Scheme 2). [2c]... 

Although the Diels-Alder cycloaddition is a highly versatile process for six-membered ring synthesis, the lack of reactivity of various dienophiles, such as unactivated alkynes, has limited its synthetic scope. Several examples of Diels-Alder reactions involving unactivated alkynes have been reported for which the reaction proceeds extremely slowly or is entirely unsuccessful. To avoid elevated temperatures, the feasibility of using nickel(O) to catalyze the cycloaddition under milder conditions was explored. The results of this investigation have been fruitful. For example, in the presence of 10 mol% Ni(COD)2 and 30 mol% tri-o-biphenyl phosphite in THF at 55 C, 11 undergoes efficient cycloaddition to provide 12 in 85% yield (Eq. 54). In contrast, in the absence of catalyst, 11 reacts only at... 

Unactivated thiophenes show little tendency to give products as 47t components in a Diels-Alder sense, for example even at 15 kbar, only maleic anhydride, of the common, electron-deficient dienophilic partners, gives an adduct." Electrophilic alkynes will react with thiophenes under vigorous conditions," though the initial adduct extrudes sulfur and substituted benzenes are obtained as products. 

The Diels-Alder reaction of oxazoles with alkynes has become a preferred method for the synthesis of substituted furans with diverse applications. A large number of early examples of this reaction have been tabulated. Activated dienophiles such as acetylenic ketones and esters can be used, although unactivated alkyl, aryl, and silyl alkynes have been used as well. In particular, the reaction of 4-phenyloxazole with substituted acetylenes is frequently used for preparing 3,4-disubstimted furans. Cycloadducts derived from 4-phenyloxazole typically decompose under milder conditions than 4-alkyloxazoles, allowing the synthesis of a wider range of functionalized furans. Several examples are shown below. 

Unactivated acetylenes are poor dienophiles and react only with difficulty with dienes to give [4 + 2] cycloadducts. This limits their use as dienophiles in the Diels-Alder reaction. To overcome this problem, several imaginative alternatives employing alkyne equivalents have been developed. Utilizing these synthetic equivalents in the Diels-Alder reaction represents an indirect solution to this reactivity dilemma be-... 

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